We already knew from thorough investigation that immediately prior to the accident, either the driver of the first vehicle changed lanes without signaling or the driver of the second vehicle was driving with excessive speed. Either of these actions would make a driver liable for the resulting accident. But further evidence has proved that the first vehicle's turn signal was not on, though the driver of that vehicle admits to having changed lanes. So the driver of the second vehicle is not liable for the accident. Which one of the following would be most important to know in evaluating the conclusion drawn above?
(A) whether the second vehicle was being driven at excessive speed
(B) whether the driver of the first vehicle knew that the turn signal was not on
(C) whether any other vehicles were involved in the accident
(D) whether the driver of the first vehicle was a reliable witness
(E) whether the driver of the second vehicle would have seen the turn signal flashing had it been on